Process of manufacturing leather and products used in connection therewith



Oct. 2, 1934. I Q s w ET AL 1,975,670

PROCESS OF MANUFACTURING LEATHER AND PRODUCTS USED IN CONNECTION THEREWITH Filed June 3, 1950 ge zwauq j :=::::::::q-, =7

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NVENTORS CHARLESGLSHAW dAcpuss HoFFMAN Patented Oct. 2, 1 934.

UNITED STATES PROCESS OF MANUFACTURING LEATHER AND PRODUCTS USED IN CONNECTION THEREWITH Charles G. Shaw, Huntsville, and Jacques Hollman, Toronto, Ontario, Canada Application June 3, 1930, Serial No. 459,103 In Canada November 29, 1929 6 Claims.

' This invention relates to a process of applying coating materials to sole leather and more particularly to the application of such coating materials subsequently to the tanning and filling of the leather, but prior to the mechanical operations of smoothing and finishingthe leather.

Many attempts have been made to perfect the appearance and color of sole leather by the application of coating materials and many bleaching processes are in common use. These bleaching processes are generally considered harmful to the leather. Other bleaching processes consist in the precipitation upon the leather of a substance having the eflect of a pigment, such as lead sulphate precipitated from lead acetate, and boracic acid precipitated from borax. These coating and bleaching processes all have numerous disadvantages, the principal one being the impermanence of the coloring effect. Many pigmented or colored coatings come off due to improper adhesion to the leather. This condition is especially aggravated by the re-soaking and tempering in water to which sole leather is generally subjected at the shoe factory. As sole leather, produced by first quality tannages frequently contains twenty-five to thirty percent of water soluble matter, and as the leather in many shoe factories is soaked in water for hours as a preliminary tempering process the result is analogous to the soaking of a painted bar of soap. Some coatings adhere fairly well but are made soft and cheesy by the soaking so that they are easily damaged.

It is obvious that such impermanent coatings on sole leather are objected to by the shoe manufacturer.

It is also obvious that if all sole leather at the tannery were coated with an absolutely soil proof, permanent and uniform finish of any color desirable, such leather would have great appeal to the shoe manufacturer as the cost of finishing and painting the soles of the shoes would be saved.

The principal object of this invention is the production of sole leather of natural and uniform appearance and of any desired color by coating the leather with suitable compounds.

A further object is to apply this coating in such a manner that it will adhere to the leather and will not be removed or damaged by the soaking or other machine operations to which the leather is subjected.

A still further object is to provide a product from which oil stains or the like may be readily removed by a simple washing operation, thus eliminating the use of acid bleaches.

These and other objects are accomplished by means of our process, which consists in the application to the leather of certain types of waterproof materials consisting of cellulose esters with or without pigments, natural or synthetic resins, drying or semi-drying oils, all dissolved in a suitable solvent.

In the application of a coating to sole leather whether it consists of one or more coats or either clear or pigmented composition it is the adhesion to the leather of the first coat which is essential. Further this adhesion must be such that it is not disturbed by prolonged subsequent soaking of the leather in water. We have discovered that regardless of the quality or properties of the coating material, the first coat must be applied to the leather when the grain surface is open, absorbent, and unglazed. This is of very great importance, and further, as the leather must be easily re-soaked by immersion in water, the films or coatings, which have a tendency to be impervious to water, must be applied in such a manner as not to cover over or plug the hair holes in the leather, which act as holes in a sieve to allow the water to go through the coating layer and to be absorbed by the leather beneath it.

In the drawing which accompanies this specification,

Figure 1 is a greatly enlarged cross sectional view of the grain portion of rough dried sole leather made according to our process.

Figure 2 is a similar cross section, showing the leather with a double coating after the compacting operation.

Figure 3 is a. cross section of a piece of sole leather similar to Figure 1, showing one of the hair holes bridged over by the coating.

In the drawing, the hyaline layer is designated by the letter H, while A designates the surface of this hyaline layer. The first or prime coating of applied material is designated by the letter B. D represents the surface coating applied upon the prime coat B. The hair holes are represented by the letter E. In Figure 2, the coating layers B and D have been applied after the compacting operation and this figure shows the hair holes E closed and sealed over at J.

In Figure 3, one of the hair holes E is shown as bridged over by the coating material extending down the sides of this small hole.

In carrying out our invention, the coating materials are applied to the sole leather prior to the mechanical finishing operations, such as setting out, jacking, rolling and pressing, which operations tend to glaze the leather surface and no oil close the hair holes After the completion of the actual tanning process, sole leather is usually taken from the tan liquors, washed, bleached by immersion in alkali and in acid solutions, wrung to partially expel the moisture and then placed in revolvingdrums with oil and suitable filling materials. The leather is then subjected to a period of drumming which incorporates the oil and fillers into the leather. In order to smooth out the folds and wrinkles, the leather should then be set out in a. setting machine in such a manner that the rotating setting cylinder should contact with and work upon the fiefsh side of the leather instead of on the grain side, as is the present custom. This setting on the flesh side facilitates subsequent coating operations and does not tend to gloss the grain of the leather. If this setting out is done on the grain side of the leather, a serious reduction in the adhesion of the coating materials will be experienced. The leather is then hung in low temperature lofts or tunnels, usually not over 100 F., and dried until the leather shows a uniformly dry color and contains not more than about twenty percent moisture by weight. It is at this stage that the coating materials are preferably applied. At least two coatings are desirable, the first or priming coat having little or no pigment or coloring matter incorporated. The subsequent coating may contain pigment or coloring matter, if desired.

As stated above, it is important to have any setting out or jacking operation, prior to coating, performed on the flesh side of the leather, instead of on the grain side. In every instance where the first coating was applied to the leather, after a setting out operation on the grain side, the adhesion of the coating to the leather was destroyed when the leather was soaked and worked as in a factory. This loss of adhesion is out of proportion to the slight gloss or finish which appears on the leather when it is set out or jacked in the usual way on the grain side. Thus, when the coating is applied in the improper sequence of the finishing operations, there is practically no adhesion, as the coating may be removed from the re-soaked leather by a rubbing with the fingers. On the other hand when the coating is applied in the proper sequence, it cannot be removed from the leather, excepting by the use of solvents.

The coating compositions should preferably be applied by means of an air spray gun, in which case a light uniform coating may be applied without permanently plugging the hair holes as indicated by the letter F in Figure 3. When the coating is properly applied, the hair holes will be unobstructed, with the coatings running down the sides of the hair holes as shown in Figure 1, but not obstructing them. If leather is coated after it is compacted and finished, that is after rolling and pressing, the coating will bridge over and plug the hair holes, as shown at J in Figure 2. This obstruction of the hair holes, is very undesirable because, under such conditions, when the leather is re-soaked, the water must penetrate through to the grain from the fiesh side leather coated in the manner shownin Figure 2 has an unnatural appearance, as-ithe hair holesare not visible. 7

The following procedure is given as an example of the method according to our invention: After J the leather is drummed and set on the flesh side and dried as previously stated; a coating of leather primer is applied by a spray gun and allowed to dry at room temperature for about one hour. This priming coat consists of the following ingredients in approximately the proportions stated; nitro cellulose 5 parts, castor oil 3 parts, boiled linseed oil 1 parts; and as solvents, ethyl acetate 35 parts, amyl acetate 8 parts and toluol 40 parts. A second or finishing coat of approximately the following composition is then sprayed onto the leather and allowed to dry at room temperature for about twelve hours; raw sienna 10 parts, pyroxylin 7 parts, castor oil 6 parts; and as solvents, butyl acetate 20 parts, ethyl-acetate 20 parts, butyl lactate 3 parts and toluol 20 parts. The leather is then put through the usual finishing operations,first dipping in water and piling down for twelve hours, then jacking, then setting out and finally pressing without rolling. These last mentioned operations and the machines for performing same are all well known in the art and need not be further described. It may be stated, however, that the setting out machine used for this purpose should preferably have hardened stainless steel blades. The blades of the jacking machine should also preferably be of Stellite metal.

, Further, in order to further facilitate the rapid resoaking of the leather through the coating and to prevent the hair holes from being collapsed and squeezed shut by the finishing and heavy compacting operations, such as jacking and pressing, we have found that a sponging fit or paste should be applied to the leather prior to such finishing operations. As an example, after coating the leather and preparatory to jacking, a jacking fit containing considerable quantities of such porous substances as clay, pigment, boracic acid, etc., is applied over the coating. The jacking works the above mentioned porous solids into the hair holes, and subsequent pressing operations cannot close the hair holes, and during resoaking the plug of porous material acts as a wick to conduct the water down the hair holes and through the coating. Boracic acid has been found to have excellent qualities for'such fits, preferably applied cold mixed with water to a soupy paste.

As a further example, leather which has been completely finished and compacted by any ordinary tanning method, but which has not been coated, may be coated so that the coating will adhere perfectly when treated as follows. The leather is first thoroughly soaked and softened by immersion in water or tan liquor for about three days. It is then washed, bleached, wrung and lightly oiled and filled in a drum. These operations remove the surface gloss and finish. The leather is then prepared to receive the coatings which may be applied when the leather is dry, as stated in the first example. We do not wish to restrict ourselves to the application of two coatings as satisfactory results may be accomplished with one coat and similarly better results may be expected with a greater number of coats. Moreover, these two coats may or may not be of similar composition. We have found it preferable to use one type of material as a primer and another type for the secondary coating. .The' time required for drying between suc- .-cessive coats varies with the composition of materials used. With some materials it is possible to apply successive coats without the necessity for drying, while with other materials, from fifteen 'minutesto twenty-four hours may be required between coatings to dry the leather properly.

Further, we do not restrict ourselves to the exact proportions of ingredients, nor to the ingredients as given in the example quoted. It is essential, however, that the coating materials should contain one of the following cellulose esters or synthetic resins. The resin is preferably of the polyhydric alcohol-polybasic acid type known as alkyd resins and drying oils or semi-drying oils may be added, together with suitable pigments, or coloring matter all suitably compounded and mixed with suitable solvents.

Moreover, the method of application may be modified. Leather coming from the oil and filling drums may be set out on the grain side, dried, soaked in any of the usual dry dip liquors and given its second oiling in a drum in the regular way, thus removing the gloss of the first setting out operation. It may then be set out on the flesh side and dried in the usual manner and at this point the priming coat and other coats, if desired, may be applied. The leather must not, however, be set out or otherwise smoothed on the grain side after the second drumming and before the first coating is applied. It is necessary to apply the first coating at the point in the process where the leather has been dried to a uniformly dry color, after having been either drummed or bleached or operated upon to open up the grain, but the leather should never under any conditions be set out or smoothed on the grain surface immediately prior to the application of the first coating. It is essential that the first or priming coat should be applied when the grain is open and absorbent and free from the slightest gloss caused by such operations as setting out, jacking, rolling, or pressing. Further coats may be applied at any point in the finishing process, but the surface of the first coat should be reasonably free from moisture and foreign substances, such as oil, soap, sugar and tanning extract.

After the first coating has been properly applied to the leather, it may then be soaked, drummed in oil, set out, and subjected to any or all of the regular finishing processes before subsequent coats are applied. There is an advantage in sub-' jecting the coated leather to such violent mechanical treatment as drumming or jacking, as this treatment does not lessen the adhesion but tends to break down the filament which may form over some hair holes, as illustrated by the letter F in Figure 3 of the drawing. Such mechanical working also tends to give the finished product a more natural appearance.

The surface of the grain of sole leather is formed of an extremely thin layer of a horny substance called the hyaline layer. True solutions of low viscosity have no difficulty in passing through this layer, but solutions of a more or less colloidal nature such as nitro cellulose, or blown oils, penetrate this layer with great difliculty. If the leather is not glossed or smoothed after the tanning process, such preparations as the prime coat have some penetration into the hyaline layer, thus producing a real bond when the coating dries. If the leather is set out or jacked, this layer is burnished so as to form a filtering surface capable of allowing the solvent to pass through, but excluding all constituents not in true solution, so that no bond is established. Without this actual penetration or bond, the coating loosens from the leather when it is soaked in .water. Our process may be used for producing permanent uniform color finishes on all types of sole leather, regardless of the type of tanning agent used.

Heretofore, coatings which were applied to sole leather and particularly vegetable tanned leather, have proven unsatisfactory either because they softened or came off when the leather was wet or cracked when the,leather was dry. These finishes also had a tendency to absorb oils or stains accidentally applied in a shoe factory. Our coating materials, applied according to our process, adhere perfectly, even though the leather is soaked for weeks. These coatings will not crack nor will they absorb oil or ink stains and, thus no precautions are necessary when this leather is worked in a shoe factory. The appearance of the leather is greatly enhanced and shoes having such coated leather soles do not stain by oxidation of tanning materials when drying in the factory. Furthermore, this leather withstands marking by customers walking on them in the shoe store. This process may also be applied to great advantage on leather used for insoles, as it allows perspiration vapors to pass through, but preventsthe staining of socks due to soluble tanning material in the leather.

Having thus described our invention, what we claim is:

1. In a process of manufacturing sole leather, the steps of applying a permanently adhesive waterproof coating, to the grain surface of sole leather while said surface is in an open, absorbent and unglazed condition and subsequently rupture the waterproof filament which may cover the hair holes of the leather, and thereby open them.

2. In a process of manufacturing sole leather,

the steps of applying to the grain surface of the leather while said surface is in an open, absorbent and unglazed condition a prime coating of waterproof material containing a cellulose ester and an oil dissolved in a suitable solvent and then applying one or more secondary coatings containing a cellulose ester, and synthetic resin and a drying oil dissolved in a suitable solventand compounded with a pigment or dry color. 3. In a process for manufacturing sole leather, the steps of applying to the unglazed grain surface of sole leather a prime coating of waterproof material consisting of a cellulose ester, an oil and a solvent, allowing said prime coating to dry and then applying one or more finishing coatings consisting of a cellulose ester, and a synthetic resin, a drying oil, a solvent and a pigment or dry color, allowing said coating to dry and then applying the usual finishing operations to the leather.

4. A process of manufacturing sole leather which includes the steps of applying a coating of waterproof material to the grain surface of previously tanned leather while said surface is in an open, absorbent and unglazed condition, ap-

plying to the coated leather a dressing contain-' ing porous substances such as clay, pigments and 'boracic acid and then subjecting the leather to a 5. A method of coating previously tanned sole leather which consists in applying the coating materials to the grain surface of the leather while said surface is in an open, absorbent and unglazed condition and then mechanically treating condition,

the leather to break down such thin filaments of coating as may have formed over the hair holes.

6. A method of coating previously tanned sole leather which consists in applying the coating materials to the grain surface of the leather while said surface is in an open, absorbent and unglazed mechanically treating the coated leather to break down such films 01' the coating as may have formed over the hair holes, then JACQUES HOFFMAN. 

